Typically, a DC/DC converter is used to convert a power supply voltage with an unstable level output from a DC power supply to a voltage with desired stable level. Since a charge pump type DC/DC converter uses capacitors as an energy storing element and needs no coil or reactor, it is small and inexpensive and has little EMI (electromagnetic interference). On the other hand, the voltage ripple is large, which is considered a problem.
FIG. 13 shows the theory of a conventional charge pump type DC/DC converter. This DC/DC converter is used for 1.5 times boosting. It has a voltage input terminal 122 connected to the output (positive) terminal of DC power supply 120, two flying capacitors Ca, Cb, capacitor Cs for smoothing, and voltage output terminal 124 connected to a load (not shown in the figure). Smoothing capacitor Cs is constantly connected between voltage output terminal 124 and the ground potential. Flying capacitors Ca, Cb switch alternately between the connection state of phase I shown in FIG. 13(A) and the connection state of phase II shown in FIG. 13(B).
More specifically, in phase I, two flying capacitors Ca, Cb are connected in series between voltage input terminal 122 and the ground potential in such a way that their positive terminals (+) face the side of voltage input terminal 122. In that connection state, two flying capacitors Ca, Cb are charged by the current flowing from DC power supply 120 to ground. In this case, when the capacitances of two flying capacitors Ca, Cb are set equal to each other, two flying capacitors Ca, Cb are charged to 0.5 Vin with respect to the output voltage of DC power supply 120, that is, the power supply voltage Vin. During that period, smoothing capacitor Cs is discharged to the load side via voltage output terminal 124 to maintain the supply of output voltage Vout to the load.
In phase II, two flying capacitors Ca, Cb are connected in parallel with each other between voltage input terminal 122 and voltage output terminal 124 in such a way that their positive terminals (+) face voltage output terminal 124. In that connection state, a voltage of 1.5 Vin obtained by adding the charged voltage 0.5 Vin of two flying capacitors Ca, Cb to the power supply voltage Vin obtained from DC power supply 120 is supplied to the load and smoothing capacitor Cs via voltage output terminal 124.
In this DC/DC converter, when phases I and II are repeated and switched alternately, as shown in FIG. 14, an output voltage Vout, which approximately has a saw tooth waveform and decreases approximately monotonically in the period of phase I and increases approximately monotonically in the period of phase II, is obtained.
FIG. 15 shows the detailed circuit configuration of a DC/DC converter. In the switch circuit network shown in the figure, N-channel MOS transistors (referred to as “NMOS transistor” hereinafter) 126, 128, and 130 receive control signal φ from a switching control circuit (not shown in the figure) at their gate terminals and are turned on during the period of phase I and turned off during the period of phase II. On the other hand, NMOS transistors 132, 134, 136, and 138 receive control signal φ−, which has a phase difference of 180° from the control signal φ, from the switching control circuit at their gate terminals and are turned off during the period of phase I and turned on during the period of phase II.
As described above, in a conventional charge pump type DC/DC converter, although two flying capacitors Ca, Cb are connected to the current path from DC power supply 120 during the period of phase I, no current path is formed between DC power supply 120 and voltage output terminal 124. The output voltageout, which is only dependent on the discharge of smoothing capacitor Cs, decreases at a relatively steep slope. As a result, a large voltage ripple occurs in the output voltageout.
An general object of the present invention is to solve the problem of the conventional technology by providing a charge pump type DC/DC converter with an improved ripple characteristic in the output voltage.